Oily mist resistant electret filter media and method for filtering

ABSTRACT

An oily mist resistant electret filter media is provided. The filter media includes polypropylene electret fibers and a melt processable fluorochemical additive. The additive has a melt temperature of at least 25° C. and a molecular weight of about 500 to 2500. Also provided is a method for filtering particulate material from air containing oily aerosol particles.

This is a continuation of application Ser. No. 08/038,145, filed Mar.26, 1993, now abandoned.

FIELD OF THE INVENTION

The invention concerns electret-enhanced filter media (more simplycalled "electret filters") made of fibers such as melt-blownmicrofibers. The invention concerns fibrous electret filters forremoving particulate matter from air and improved fibers for makingthose filters. The invention is especially concerned with respiratorsand the problem of improving the ability of filter media to acceptfiltration-enhancing electrostatic charges and to sustain that electretfiltration enhancement in the presence of oily aerosols.

DESCRIPTION OF THE RELATED ART

For many years nonwoven fibrous filter webs have been made frompolypropylene using melt-blowing apparatus of the type described inReport No. 4364 of the Naval Research Laboratories, published May 25,1954, entitled "Manufacture of Super Fine Organic Fibers" by Van Wenteet al. Such melt-blown microfiber webs continue to be in widespread usefor filtering particulate contaminants, e.g., as face masks and as waterfilters, and for other purposes, e.g., to remove oil from water.

The filtering efficiency of a melt-blown microfiber web can be improvedby a factor of two or more when the melt-blown fibers are bombarded asthey issue from the orifices with electrically charged particles such aselectrons or ions, thus making the fibrous web an electret. Similarly,the web can be made an electret by exposure to a corona after it iscollected. Melt-blown polypropylene microfibers are especially useful,while other polymers may also be used such as polycarbonates andpolyhalocarbons that may be melt-blown and have appropriatevolume-resistivities under expected environmental conditions.

Fibrous filters for removing particulate contaminants from the air arealso made from fibrillated polypropylene films. Electret filtrationenhancement can be provided by electrostatically charging the filmbefore it is fibrillated.

Common polymers such as polyesters, polycarbonates, etc. can be treatedto produce highly charged electrets but these charges are usuallyshort-lived especially under humid conditions. The electret structuresmay be films or sheets which find applications as the electrostaticelement in electro-acoustic devices such as microphones, headphones andspeakers and in dust particle control, high voltage electrostaticgenerators, electrostatic recorders and other applications.

Fibrous polypropylene electret filters that are currently available,some made from melt-blown polypropylene microfibers and others fromfibrillated polypropylene film, can show thermally stable electretfiltration enhancement.

Unfortunately, fibrous electret filters made of polypropylene, whethermelt-blown microfibers or fibrillated film, tend to lose their electretenhanced filtration efficiency faster than desired for some purposeswhen exposed to oily aerosols. There is a growing awareness of the needto improve the long-term efficiency of air filters in the presence ofaerosol oils, especially in respirators. It is known to blend about 1 to20 weight percent poly(4-methyl-1-pentene) with polypropylene to provideresistance to loss of electret enhanced filtration efficiency onexposure to oily aerosols.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides oily mist resistantelectret filter media comprising polypropylene electret fibers and amelt processable fluorochemical additive, said additive having a meltingtemperature of at least 25° C. and a molecular weight of 500 to 2500.The fibers may be in the form of meltblown microfibers. Preferredfluorochemical additives are fluorochemical oxazolidinones,fluorochemical piperazines or perfluorinated alkanes.

In another aspect, the present invention provides a method for filteringparticulate material from air containing oily aerosol particlescomprising passing said air through electret filter media comprisingpolypropylene melt blown microfibers and a melt processablefluorochemical additive.

The electret filter media of the present invention have improvedelectret filtration enhancement and sustain that enhancement uponexposure to oily aerosols. Furthermore, the electret filter media of thepresent invention maintain functional filtration enhancing charge levelsunder accelerated aging conditions.

The novel fibrous electret filter is especially useful as an air filterelement of a respirator such as a face mask or for such purposes asheating, ventilation, and air-conditioning. In respirator uses, thenovel electret filters may be in the form of molded or folded half-facemasks, replaceable cartridges or canisters, or prefilters. In such uses,an air filter element of the invention is surprisingly effective forremoving oily aerosols such as in cigarette smoke or in fumes fromcombustion engines. When used as an air filter, such as in a respirator,the electret filter media has surprisingly better filtration performancethan does a comparable electret filter made of 100% polypropylenefibers.

DETAILED DESCRIPTION OF THE INVENTION

The melt blown polypropylene microfibers useful in the present inventioncan be prepared as described in Van Wente, A., "Superfine ThermoplasticFibers," Industrial Engineering Chemistry, vol. 48, pp. 1342-1346 and inReport No. 4364 of the Naval Research Laboratories, published May 25,1954, entitled "Manufacture of Super Fine Organic Fibers" by Van Wenteet al. or from microfiber webs containing particulate matter such asthose disclosed, for example, in U.S. Pat. Nos. 3,971,373 (Braun),4,100,324 (Anderson) and 4,429,001 (Kolpin et al.), which patents areincorporated herein by reference.

The polypropylene resin used to form the melt blown microfibers shouldbe substantially free from materials such as antistatic agents whichcould increase the electrical conductivity or otherwise interfere withthe ability of the fibers to accept and hold electrostatic charges.

Blown microfibers for fibrous electret filters of the inventiontypically have an effective fiber diameter of from about 5 to 30micrometers preferably from about 7 to 10 micrometers, as calculatedaccording to the method set forth in Davies, C. N., "The Separation ofAirborne Dust and Particles," Institution of Mechanical Engineers,London, Proceedings 1B, 1952.

The fluorochemical additives useful in the present invention arefluorochemical compounds which can provide oil and water repellency tofibers. The fluorochemical additives must be melt processable, i.e.,suffer substantially no degradation under the melt processing conditionsused to form the microfibers. The fluorochemical additive is solid at25° C. and preferably has a melting point of at least about 70° C., morepreferably at least about 100° C. The fluorochemical additive preferablyexhibits no phase transitions in the range of commonly encounteredtemperatures, i.e., about 0° C. to 80° C. as such changes in molecularfreedom can adversely affect charge stability. The fluorochemicaladditive preferably has a molecular weight in the range of about 500 to2500, more preferably in the range of about 800 to 1500. Thefluorochemical additive is preferably substantially free from mobilepolar and/or ionic species, contaminants and impurities which couldincrease the electrical conductivity or otherwise interfere with theability of the fibers to accept and hold electrostatic charges.

Preferred fluorochemical additives include, for example, fluorochemicaloxazolidinones which are described in U.S. Pat. No. 5,025,052 (Crater etal.), fluorochemical piperazines which are described in Katritzky, AlanR. et al., "Design and Synthesis of Novel Fluorinated Surfactants forHydrocarbon Subphases," Langmuir, vol. 4, pp. 732-735, 1988, andperfluorinated alkanes preferably having about 10 to 50 carbon atoms,more preferably about 15 to 30 carbon atoms. The fluorochemical additiveis preferably present in amounts of about 0.2 to 10 weight percent, morepreferably about 0.5 to 5 weight percent, most preferably about 0.5 to 2weight percent.

Preferably, the filter media is annealed, i.e., heated for a sufficienttime at a sufficient temperature to cause the fluorochemical additive tobloom to the surface of the fibers. Generally, about 1 to 10 minutes atabout 140° C. is sufficient although shorter times may be used at highertemperatures and longer times may be required at lower temperatures.

The electret filter media of the present invention preferably has abasis weight in the range of about 10 to 500 g/m², more preferably about10 to 100 g/m². In making melt-blown microfiber webs, the basis weightcan be controlled, for example, by changing either the collector speedor the die throughput. The thickness of the filter media is preferablyabout 0.25 to 20 mm, more preferably about 0.5 to 2 mm. The electretfilter media and the polypropylene resin from which it is producedshould not be subjected to any unnecessary treatment which mightincrease its electrical conductivity, e.g., exposure to gamma rays,ultraviolet irradiation, pyrolysis, oxidation, etc.

The melt-blown microfibers or fibrillated fibers of the electret filtersof the invention can be electrostatically charged by a process describedin U.S. Pat. Nos. Re. 30,782 (van Turnhout) or Re. 31,285 (van Turnhout)or by other conventional methods for charging or polarizing electrets,e.g., by a process of U.S. Pat. Nos. 4,375,718 (Wadsworth et al.);4,588,537 (Klasse et al.); or 4,592,815 (Nakao). In general, thecharging process involves subjecting the material to corona discharge orpulsed high voltage.

In the following examples, all percentages and parts are by weightunless otherwise noted. The fluorochemical additives used in theexamples were as follows:

Additive A

A fluorochemical oxazolidinone was prepared following the procedure ofExample 5 of U.S. Pat. No. 5,099,026 which is incorporated herein byreference. The solid product was ground to form a powder and one partwas added to four parts refluxing solvent of 95% ethanol and 5% waterand refluxed for about ten minutes. The resulting slurry was cooled andthe solid filtered and dried at 71° C. The resulting solid had a meltingpoint of 197° C. as determined by DSC. The structure is set forth inTable I.

Additive B

Additive B was prepared using the procedure used for Additive A exceptthe isocyanate used was p-xylene diisocyanate. The resulting solidproduct had a melting point of 220° C. as determined by DSC. Thestructure is set forth in Table I.

Additive C

A fluorochemical oxazolidinone was prepared following the procedure ofExample 1 of U.S. Pat. No. 5,099,026. One part of the solid product wasdissolved in five parts dimethyl formamide at about 100° C. and theresulting solution was allowed to cool. The solid which formed wasisolated by filtration and added to 16 times its weight of refluxingethyl acetate. After cooling, the resulting solid was isolated byfiltration and dried. The resulting solid product had a melting point of169° C. as determined by DSC. The structure is set forth in Table I.

Additive D

Additive D was prepared using the procedure used for Additive A exceptthe isocyanate used was octadecyl isocyanate. The product wasrecrystallized from ethyl acetate four times. The resulting solidproduct had a melting point of 145° C. as determined by DSC. Thestructure is set forth in Table I.

Additive E

To a 500 mL three-neck flask fitted with a thermometer, drying tube andoverhead stirrer were added isophorone diisocyanate (148.7 g, 0.669 mol)and 4 drops dibutyl tin dilaurate. The reaction mixture was stirred andpentaerythritol (22.7 g, 0.167 mol) was added. The reaction mixture washeated to about 78° C. and the reaction was carried out for about 20hours during which time the viscosity of the reaction mixture increased.Ethyl acetate (148 mL) was added to the reaction mixture in portions toreduce the viscosity of the material. A condenser was fitted to theflask. Heating was discontinued when titration with n-butyl amineindicated that about 50% of the isocyanate had reacted. The finalconcentration of the resulting polyfunctional isocyanate intermediatewas about 50% in ethyl acetate.

Additive E was then prepared using the procedure used for Additive Aexcept the isocyanate used was the polyfunctional isocyanateintermediate prepared above. The reaction product was difficult tofilter. Filter cakes of the product swollen with ethyl acetate wereadded to a solution of 70% acetone and 30% water. A white solid rapidlysettled out and was isolated by filtration. The product was dried at 71°C. The resulting solid had a melting point of 182° C. as determined byDSC. The structure is set forth in Table I.

Additive F

Additive F was prepared by adding to a 3-neck round bottom 500 ml flaskequipped with a nitrogen inlet and a magnetic stirrerN-(perfluorooctylsulfonyl)-piperazine (34.1 g, 60 mmol), triethylamine(6.7 g, 66 mmol)and chloroform (200 mL) and the mixture was stirred.Phthaloyl dichloride (95%, 6.4 g, 30 mmol) was added dropwise as achloroform solution. After the addition was complete, the reactionmixture was stirred under nitrogen atmosphere for 30 minutes. Thereaction product was washed with deionized water several times. Thesolid was allowed to air dry and then was oven dried at 105° C. forthree hours. The solid product was ground to form a powder and one partwas added to four parts refluxing solvent of 95% ethanol and 5% waterand refluxed for about ten minutes. The resulting solid product wasdried at 71° C. The product had a melting point of 191° C. The structurewas confirmed by NMR. The structure is set forth in Table I.

Additive G

Additive G was C₂₄ F₅₀, available from Aldrich Chemical Co., meltingpoint 188°-190° C.

Additive H

To polypropylene resin (ESCORENE PP-3085, available from Exxon Corp.)was added about 6-7% n-methylperfluorooctylsulfonamidoethyl acrylate andperoxide initiators (0.01 mole percent LUPERSOL 101 and 0.01 molepercent LUPERSOL 130, available from Pennwalt Chemical Co.). A reactionwas carried out in a corotating twin screw extruder to graft then-methylperfluorooctylsulfonamidoethyl acrylate to the polypropylene.The reaction temperature was 220° C. and residence time was 6 minutes.

Additive I

N-methyl-perfluorooctanesulfonamide (450 g) was placed in a two-liter,three-necked, round bottom flask and heated to 80° C. Epichlorohydrin(101 g) was then added, followed by addition of methanol (91 g). Thetemperature was reduced to 65° C. and sodium methoxide (30 g, 25% inmethanol) was slowly added while the temperature was maintained below70° C. Then sodium hydroxide (60 g, 50% aqueous solution) was slowlyadded with the temperature maintained below 70° C. The resultingreaction was stirred overnight at 65° C. Vacuum was then applied toremove excess methanol and epichlorohydrin. The resulting product,N-methyl-N-glycidyl-perfluorooctanesulfonamide, was then washed twicewith water (450 g) at 65° C. Vacuum (20 mm Hg) was applied and thetemperature raised to 90° C. to remove any volatile materials.

N-methyl-N-glycidyl-perfluorooctanesulfonamide, (250 g, 0.44 mol) andtoluene (250 g) were placed in a one-liter, three-necked, round bottomflask equipped with a stirrer, condenser, gas inlet tube, thermometerand electric heating mantle under a nitrogen atmosphere. The solutionwas stirred, heated to 60° C. and then octadecylamine (118.4 g, 0.44mol) was added in small portions over 15 minutes. The temperature wasthen raised to 115° C. and the mixture was stirred for 12 hours untilgas chromatographic analysis confirmed the epoxide groups had beenconverted to aminoalcohol groups. The reaction mixture was cooled to 25°C. and excess toluene was removed under vacuum with a rotary evaporator.The resulting material was recrystallized from a solution of 5 partswater and 95 parts ethyl alcohol and had a melting point of 63° C. asdetermined by DSC analysis.

Additive J

Additive J is FS-1265 Fluid, a trimethyl terminatedtrifluoropropylmethyl polysiloxane, viscosity 10,000 CST available fromDow Corning Corp.

                                      TABLE I                                     __________________________________________________________________________    Additive                                                                           Structure                                                                __________________________________________________________________________          ##STR1##                                                                B                                                                                   ##STR2##                                                                C                                                                                   ##STR3##                                                                D                                                                                   ##STR4##                                                                E                                                                                   ##STR5##                                                                F                                                                                   ##STR6##                                                                I    C.sub.8 F.sub.17 SO.sub.2 N(CH.sub.3)CH.sub.2 CH(OH)CHNHC.sub.18              H.sub.37                                                                 __________________________________________________________________________

EXAMPLE 1

In Example 1, polypropylene (99%, ESCORENE PP-3505, available from ExxonCorporation) and Additive A (1%) were dry blended and extruded asdescribed in Van Wente, A., "Superfine Thermoplastic Fibers," IndustrialEngineering Chemistry, vol. 48, pp. 1342-1346 at a melt temperature of297° C. to form melt blown microfiber web having a basis weight of 54g/m² and a thickness of 0.79 mm. The web was annealed at 140° C. for 10minutes and then corona charged using a high voltage electric fieldprovided between a corona source and a ground electrode with a coronacurrent of about 0.01 milliamp per centimeter of corona source. Examples2-7 and Comparative Examples C1-C4

In Examples 2-7 and Comparative Examples C1-C3 webs were prepared usingthe procedure of Example 1. Comparative Example C4 contained noadditive. The additive, amount of additive, melt temperature, basisweight, thickness and ΔP are set forth in Table II. The ΔP was measuredwith a flow rate of 32 L/min at a face velocity of 5.2 cm/sec.

                  TABLE II                                                        ______________________________________                                                      Addi-                                                                         tive     Melt  Basis Thick-                                                   Amount   Temp. Weight                                                                              ness  Δp                             Ex.  Additive (%)      (°C.)                                                                        (g/m.sup.2)                                                                         (mm)  (mmH.sub.2 O)                        ______________________________________                                        1    A        1        297   54    0.79  1.2                                  2    B        1        297   54    0.74  1.2                                  3    C        1        307   54    0.99  1.7                                  4    D        1        303   50    --    1.5                                  5    E        1        280   57    0.94  1.2                                  6    F        1        307   52    1.45  2.1                                  7    G        1        307   53    1.22  1.1                                  C1   H        20       282   55    0.94  1.7                                  C2   I        1        287   54    0.99  1.5                                  C3   J        1        287   54    0.91  1.6                                  C4   --       --       301   54    0.69  1.4                                  ______________________________________                                    

The samples were then tested using the following tests:

DOP Penetration and Pressure Drop

Dioctyl phthalate (DOP) 0.3 micrometer diameter particles at aconcentration of between 70 and 110 mg/m³ are generated using a TSI No.212 sprayer with four orifices and 30 psi clean air. The particles areforced through a sample of filter media which is 4.5 inches in diameterat a rate of 42.5 L/min. at a face velocity of 6.9 centimeters persecond. The sample was exposed to the aerosol for 30 to 60 seconds untilthe readings stabilized. The penetration is measured with an opticalscattering chamber, Percent Penetration Meter Model TPA-8F availablefrom Air Techniques Inc. The DOP penetration is preferably less thanabout 40%, more preferably less than about 25%. The pressure drop ismeasured at a flow rate of 42.5 L/min and a face velocity of 6.9 cm/secusing an electronic manometer. Pressure drop is reported as ΔP in mm ofwater. Preferably the pressure drop is less than about 4, morepreferably less than about 3.

The penetration and pressure drop are used to calculate a quality value"QF value" from the natural log (ln) of the DOP penetration by thefollowing formula: ##EQU1## A higher initial QF value indicates betterinitial filtration performance. Decreased QF values effectivelycorrelate with decreased filtration performance. Generally a QF value ofat least about 0.25 is preferred, a value of at least about 0.4 is morepreferred and a value of at least about 0.5 is most preferred. Thefilter media are preferably thermally stable, i.e., show no more than30% loss in QF value after three days storage at 60° C.,

DOP Loading Test

For the DOP Loading Test, the same test equipment was used as in thepenetration and pressure drop tests. The test sample was weighed andthen exposed to the DOP aerosol for 45 min. to provide an exposure ofbetween 130 and 200 mg. DOP penetration and pressure drop are measuredthroughout the test at least as frequently as once per minute. The massof DOP collected divided by the mass of sample exposed was calculatedfor each measurement interval from the measured penetration, mass of thefilter web and total mass of DOP collected on the filter web duringexposure.

DOP penetration, pressure drop and loading tests were performed afterabout three days storage at ambient conditions after charging. Theinitial DOP penetration and pressure drop were measured after one minuteexposure to allow the system to equilibrate. The DOP penetration andpressure drop results are also interpolated to 50 and 100 mg exposure.The results are set forth in Table III.

                  TABLE III                                                       ______________________________________                                        Initial           50 mg           100 mg                                      Ex.  PEN      ΔP                                                                             QF     PEN  ΔP                                                                             PEN  ΔP                         ______________________________________                                        1    29       1.8    0.70   35   2.0    38   2.1                              2    29       1.8    0.71   33   2.0    35   2.1                              3    21       2.3    0.66   24   2.6    26   2.8                              4    15       2.2    0.87   23   2.3    33   2.4                              5    32       1.7    0.67   36   1.9    41   2.0                              6    11       4.1    0.54   22   4.3    40   4.3                              7    37       1.7    0.58   55   1.8    64   1.8                              C1   78       2.4    0.10   --   --     --   --                               C2   40       2.2    0.41   38   2.4    48   2.5                              C3   34       2.5    0.43   78   2.5    81   2.6                              C4   41       1.9    0.47   77   1.9    81   1.9                              ______________________________________                                    

Samples were aged at elevated temperature and again tested. The testresults for the aged samples are set forth in Table IV.

                                      TABLE IV                                    __________________________________________________________________________    Aging  Aging                                                                  Temp.  Time                                                                              Initial    50 mg  100 mg                                           Ex.                                                                              (°C.)                                                                      (hrs)                                                                             Pen ΔP                                                                         QF  Pen                                                                              ΔP                                                                          Pen                                                                              ΔP                                                                          QF                                        __________________________________________________________________________    1  60  72  30  1.8                                                                              0.67                                                                              32 2.0 32 2.2 0.52                                      2  60  72  27  1.8                                                                              0.73                                                                              29 2.0 30 2.2 0.55                                      3  60  73  38  2.1                                                                              0.45                                                                              35 2.3 33 2.5 0.44                                      4  60  47  20  2.2                                                                              0.72                                                                              22 2.4 29 2.4 0.52                                      5  70  70  38  1.7                                                                              0.57                                                                              39 1.9 40 2.0 0.46                                      6  60  73  17  3.6                                                                              0.48                                                                              24 3.8 34 3.9 0.28                                      7  60  73  38  1.6                                                                              0.61                                                                              44 1.7 57 1.7 0.33                                      C1 60  24  83  2.4                                                                              0.08                                                                              -- --  -- --                                            C2 70  72  83  2.1                                                                              0.09                                                                              81 2.1 83 2.2 0.08                                      C3 70  48  50  2.3                                                                              0.30                                                                              77 2.3 79 2.4 0.10                                      C4 60  72  35  1.8                                                                              0.57                                                                              66 1.9 76 1.9 0.14                                      __________________________________________________________________________

As can be seen from the data in Tables III and IV, addition of anappropriate melt processable fluorochemical additive to polypropyleneenhances the resistance of the filter media to damage by an oilyaerosol. In Examples 1-7, each of the materials tested had QF values ofat least 0.5 on initial testing. In the loading test, Examples 1-6 hadpenetrations less than 50% even after exposure to 100 mg DOP. AlthoughExample 7 had a penetration in excess of 50% at 100 mg DOP exposure,performance was adequate. The filter media of Comparative Example C1 didnot retain charge, possibly due to the reaction byproducts in thegrafting process or the presence of polar groups. Subsequent to aging,each of Examples 1-7 exhibit a QF value greater than 0.4 and each ofExamples 1-6 have penetrations lower than 50% after 100 mg DOP exposure.Comparative Example C2 did not retain charge after elevated temperatureaging, possibly due to the low melting point of the fluorochemicaladditive and the presence of polar groups in the molecule.

Comparative Examples C5-C7

Samples were prepared as in Example 1 except in Comparative Example C5the web was not charged, in Comparative Example C6 the web was notannealed, and in Comparative Example C7 the web contained no additive,was annealed and charged. The webs were tested as were the webs ofExamples 1-7. The results are set forth in Table V together with theresults of Example 1 tests for comparison.

                  TABLE V                                                         ______________________________________                                        Initial           50 mg           100 mg                                      Ex.  PEN      ΔP                                                                             QF     PEN  ΔP                                                                             PEN  ΔP                         ______________________________________                                        1    29       1.8    0.70   35   2.0    38   2.1                              C5   84       1.9    0.09   81   2.0    79   2.0                              C6   30       2.1    0.58   69   2.1    81   2.1                              C7   41       1.9    0.47   77   1.9    81   1.9                              ______________________________________                                    

As can be seen from the results in Table V, the fluorochemical additive,the annealing and the charging are each important in the practice of thepresent invention.

Examples 8-10 and Comparative Example C8

In Examples 8-10, samples were prepared as in Example 1 using 0.5, 1 and2 weight percent, respectively of Additive A. In Comparative Example C8no additive was used. The samples were stored at 70° C. for 72 hours.Testing was carried out as in the previous examples. The results are setforth in Table VI.

                  TABLE VI                                                        ______________________________________                                        Initial           50 mg           100 mg                                      Ex.  PEN      ΔP                                                                             QF     PEN  ΔP                                                                             PEN  ΔP                         ______________________________________                                        C8   44       2.2    0.4    70   2.1    83   2.3                              8    30       2.3    0.5    32   2.5    38   2.6                              9    31       2.1    0.6    28   2.4    27   2.6                              10   39       1.7    0.5    39   1.9    37   2.1                              ______________________________________                                    

As can be seen from the data in Table VI, at addition levels of only 0.5weight percent, Additive A provides improved performance.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention and this invention should not be restricted to thatset forth herein for illustrative purposes.

What is claimed is:
 1. An oily mist resistant electret filter mediacomprising melt blown polypropylene electret microfibers and a meltprocessable fluorochemical additive compound having a melting point ofat least about 25° C. and a molecular weight of about 500 to 2500present in an amount of about 0.2 to 10 weight percent to achievesustained electret filtration enhancement.
 2. The filter media of claim1 wherein the fluorochemical additive is a fluorochemical oxazolidinone,fluorochemical piperazine or perfluorinated alkane.
 3. The filter mediaof claim 1 wherein said additive has a molecular weight of about 800 to1500.
 4. The filter media of claim 1 wherein said additive has a meltingpoint of at least about 70° C.
 5. The filter media of claim 1 whereinsaid additive is substantially free of mobile polar species.
 6. Thefilter media of claim 1 wherein said additive is substantially free ofmobile ionic species.
 7. The filter media of claim 1 wherein saidmicrofibers have an effective fiber diameter of about 5 to 30micrometers.
 8. The filter media of claim 1 wherein said fibers are meltblown microfibers.
 9. The filter media of claim 1 wherein said media hasa basis weight of 10 to 100 g/m².
 10. The filter media of claim 1wherein said media has a thickness of about 0.25 to 20 mm.
 11. A methodfor filtering particulate material from air containing oily aerosolparticles comprising passing said air through electret filter mediacomprising polypropylene melt blown microfibers and a melt processablefluorochemical additive, said additive having a melting point of atleast 25° C. and a molecular weight of about 500 to 2500 present in anamount of about 0.2 to 10 weight percent to achieve sustained electretfiltration enhancement.
 12. The method of claim 11 wherein thefluorochemical additive is a fluorochemical oxazolidinone,fluorochemical piperazine, or perfluorinated alkane.
 13. The method ofclaim 11 wherein said additive has a molecular weight of about 800 to1500.
 14. The method of claim 11 wherein said additive has a meltingpoint of at least about 70° C.
 15. The method of claim 11 wherein saidadditive is substantially free of mobile polar species.
 16. The methodof claim 11 wherein said additive is substantially free of mobile ionicspecies.
 17. The method of claim 11 wherein said microfibers have aneffective fiber diameter of about 5 to 30 micrometers.
 18. The method ofclaim 11 wherein said fibers are melt blown microfibers.
 19. The methodof claim 11 wherein said media has a basis weight of 10 to 100 g/m². 20.The method of claim 11 wherein said media has a thickness of about 0.25to 20 mm.